Optical computing, optical switching, and optical interconnection are three emerging areas in which it is necessary to modulate optical beams. In the optical computing and optical interconnection fields, a high degree of interest has been generated by semiconductor devices and arrays which operate on light beams propagating normal to the surface plane of the device or array. Such devices are commonly described as "surface-normal" devices. Interest in surface-normal devices and arrays is high both because the devices are relatively compact which permits array fabrication and because optical coupling to and from the devices is effected in a simple and convenient fashion.
With respect to optical modulators, a wide variety of surface-normal optical modulators have appeared recently. Among the first surface-normal optical modulators were those based on electroabsorption in semiconductor quantum wells. That is, optical beams impinging on the surface of the modulator were either absorbed by the semiconductor quantum well or permitted to pass through the modulator without significant absorption in response to electrically induced changes in the optical absorption coefficient of the semiconductor material. As a result, the optical signal output from the modulator was a optical version of the electrical signal impinging on the modulator. An example of a high speed, surface-normal optical modulator based on the principles of electroabsorption and employing semiconductor quantum well material is shown in U.S. Pat. No. 4,525,687.
Temperature sensitivities of modulators employing electroabsorption detract to some degree from their appeal to device and system designers. When these modulators absorb the optical beam, the absorption process causes the modulator to undergo increasing thermodynamic effects. This is particularly deleterious for semiconductor quantum well electroabsorption modulators because the optical beams are tuned to a wavelength near the absorption band edge of the quantum well material. The absorption edge is extremely sensitive to temperature changes. Heating in the modulator through the absorption process induces a shift in the absorption edge in the semiconductor quantum well material so that the wavelength of the optical beam is no longer aligned with the absorption edge of the modulator. As a result, the modulator is rendered incapable of modulating the optical beam. Another related problem for electroabsorption modulators arises from electrical carrier production during modulation caused by absorption. Electrical carriers tend to screen an applied electrical field and, thereby, cause modulation to cease when carrier populations approach a sufficiently high level. Carrier production also tends to decrease the optical absorption coefficient of the semiconductor material. This limits the applicability of the semiconductor electroabsorption modulators to operation on beams which exhibit low optical intensity.
In contrast to electroabsorption modulators, optical modulators have been designed to utilize refractive index changes of the semiconductor material for controlling transmission of optical beams through the modulator. Since these devices do not entail optical absorption processes, the problems described above for electroabsorption modulators are avoided. Optical modulators employing refractive index changes do not absorb the impinging optical beams. As a result, thermal and electrical effects are avoided in the modulation process and high intensity modulation is thereby permitted. In addition, modulators employing refractive index changes appear more useful for the construction of systems because the modulator is switched between transmission and reflection states. There is effectively no loss of the optical beam in the modulator. As a result, modulators employing refractive index changes can be read by two detectors, namely, one detector measuring reflected optical signals and the other detector measuring transmitted optical signals, which in turn leads to increased system flexibility.
Prior attempts at realizing surface-normal modulators employing refractive index changes have achieved only moderate success because the modulation depth is, to a first approximation, proportional to the product of the refractive index change of the semiconductor material and the thickness of the semiconductor material. Refractive index changes depend upon the choice of the semiconductor material and the size of the electrical field which may be impressed on the semiconductor material. From the available technical articles, it is determined that thicknesses for semiconductor material generally used in such refractive index modulators are typically five to ten times larger than semiconductor material thicknesses in absorptive modulators which provide similar modulation performance. Typical semiconductor refractive index modulators are described in Applied Physics Letters, Vol. 51, No. 23, pp. 1876-8, (1987) and Applied Physics Letters, Vol. 53, No. 8, pp. 637-9, (1988). As a practical matter for surface-normal modulators, it is desirable to orient the controlling electric field in a direction perpendicular to the semiconductor layers of the modulator. Applied voltages for reported surface-normal refractive index modulators are significantly higher than those for reported surface-normal electroabsorption modulators when performing comparable levels of modulation.
The refractive index modulator described in the 1987 article cited above consists of a dielectric mirror consisting of thirty periods of GaAs and AlAs semiconductor layers whose individual layer thicknesses are equal to a predetermined wavelength divided by the product of four and the respective refractive index. Such a mirror is highly reflecting for a range of wavelengths around the predetermined wavelength. At the edge of the range, the modulator abruptly becomes transmitting. An electrical field is applied perpendicular to the layers of the mirror via contacts at the top and bottom of the mirror. Application of an electric field to the mirror induces changes in the refractive index of the GaAs layers which, in turn, causes the reflection characteristic of the mirror to shift position slightly. Optical beams tuned near the edge of the reflection range for the mirror experience the shift of the reflection characteristic are modulated thereby. In order to obtain a modest degree of modulation, it was necessary to operate the device with an applied voltage on the order of 50 to 100 volts. Such levels of applied voltage are prohibitively high and, therefore, undesirable for optical computing and optical interconnection applications.